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Kennecott Greens Creek Mine finished a $5 million mill expansion project earlier this month. The mine brought the expanded system on line Jan. 15.
The company added 33 flotation cells and built a 4,800-square-foot building to house them. The cells are where target minerals other than gold are "boiled" to the surface and collected for more processing into a salable product.
The expansion project, which added around 50 percent to the mine's flotation cell capacity, came in roughly $500,000 under budget and about two months ahead of schedule, said mine manager Keith Marshall.
"And we didn't lose a day of production either," he said.
The new building sits between the mine's ore pile, a couple of hundred yards outside the entrance to the mine, and the ore grinding and concentrating area.
Greens Creek Mine, about 20 air miles from downtown Juneau on Admiralty Island, is one of the largest silver producers in North America. It is jointly owned by Salt Lake City-based Kennecott Minerals Co. with a 70 percent stake and Hecla Mining Co. out of Coeur d'Alene, Idaho., with the remaining ownership share.
The mine's ore body was discovered in 1975 with exploration beginning three years later. Development started in 1987 with concentrate production beginning two years later. Low metal prices closed the operation in 1993. Since reopening in 1996, it has grown to become the largest private employer in Juneau with about 270 people on its payroll.
The mine has about 10 million tons of ore reserves, or around 16 years worth at the present rate of mining, Marshall said.
Greens Creek sells four products: gold bars, concentrates of lead and zinc, and a bulk concentrate. The concentrates are shipped to smelters all over the world.
The lead concentrate contains the silver making it the most valuable of the three concentrates. Bulk concentrate contains lead and zinc as well as other minerals, but is of a lower grade and not worth as much per ton as the others, Marshall said.
Most of the mine's income comes from silver and zinc, each accounting for about 40 percent, with gold picking up 15 percent and lead 5 percent, Marshall said.
The flotation cells are a critical part of turning ore into a marketable product. Ore is first crushed to a fine powder and run through a system to collect and concentrate gold. The ore is then mixed with water and chemicals in the cells and run though in two stages.
The chemicals make zinc or lead hydrophobic, or "afraid" of water. Air is then bubbled through the mixture in the cells. The targeted mineral attaches to the air bubbles and goes to the surface where the bubbles overflow a cell's interior wall like shiny metallic lava.
Bob Haecker, a metallurgical engineer for the mine, said the first stage is rougher cells that are like a first pass to catch as much of the target metal as possible. The second stage is cleaner cells that take what the rougher cells collected and get rid of more waste rock, further increasing the concentrate's grade and value.
The mine's new 10-foot-by-11-foot cells are cleaners, 16 for lead and 17 for zinc.
"They're the same size (as the original cells), there are just more of them," Haecker said.
It will take roughly to the end of the month before the machines are running at peak efficiency, Marshall said.
The mine's mill was originally designed to handle 800 to 1,000 tons of material a day. It now handles between 1,700 and 1,800 tons a day.
The cells won't increase the throughput tonnage of the mine, but instead increases the amount of time it takes to process the ore from three to four-and-a-half hours, Marshall said. More throughput time increases the efficiency of the concentrator cells and increases the recovery percentage of metals, he said.
In 1999, Greens Creek milled 573,000 tons of ore with an average grade of 4 ounces of silver per ton, 14 percent zinc, 0.2 ounces of gold per ton and 6 percent lead. The concentrator averaged 1,580 tons per day and produced 77,000 tons of zinc concentrate, 42,000 tons of lead concentrate and 71,000 tons of bulk concentrate.
Last year, the mine milled 619,000 tons of ore, 46,000 more tons but of a lower quality than 1999. The increased tonnage was a result of the mine increasing its efficiency, he said.
"That will continue," Marshall said, but a mine expansion isn't in the works.
For now, Marshall feels good about the mine's operations.
"Oh, we're having a great month," he said.
Greens Creek hit an ore body that is running about 200 ounces of silver per ton. That will run out soon, he said, but it's a great way to start the year.
Mike Hinman can be reached at firstname.lastname@example.org.